This invention relates to plasticating using a screw rotatable within a barrel to extrude or inject molten resinous material. More particularly, this invention is concerned with improvements in melting and mixing the resinous material using a screw having a material reorientation section between a barrier melt section and an undulating metering section.
A plasticating apparatus commonly used today receives polymer or thermoplastic resin pellets, granules or powders, from an inlet port, then heats and works the resin to convert it into a melted or molten state. The melt or molten material is delivered under pressure through a restricted outlet or discharge port to make the finished article. It is desirable that the molten material leaving the apparatus be completely melted and homogeneously mixed, resulting in uniform temperature, viscosity, color and composition.
Typically, the basic plasticating apparatus has an elongated cylindrical barrel which is heated at various locations along its length. An axially supported and rotating screw extends longitudinally through the barrel. The screw is responsible for forwarding, melting, pressurizing and homogenizing the material as it passes from the inlet port to the outlet port. Typically, the screw has a core with a helical flight thereon and the flight cooperates with the cylindrical inner surface of the barrel to define a helical valley for forward passage of the resin to the outlet port.
There are several different types of thermoplastic resins or polymers, and each has different physical properties and characteristics. Therefore, there are several different screw configurations. In general, however, the typical plasticating screw has a plurality of sections along its extended axis with each section being designed for a particular function. Ordinarily, there is a feed section, a melting section and a metering section in series. In the art, the melting section has been referred to interchangeably as the intermediate, compression or transition section.
The feed section extends forward from the inlet port of feed opening where solid thermoplastic resins, in pellet, granular or powder form, are introduced into the apparatus and pushed forward by the screw along the inside of the barrel. The resin is then worked and heated in the melting section. After approximately 40 to 80 percent of the resin has been melted, solid bed breakup occurs, and solids become randomly dispersed within the melt. It is important to note that most melting initially occurring in the melting section takes place at or near the heat source of the inner wall of the barrel.
To assure a homogeneous melt, therefore, it is often important that solid material be separated from the molten material in the melting section using a barrier to create two adjacent helical channels, a solids channel and melt channel with a barrier flight therebetween, so that the thin melt film which develops at the outer periphery of the solids channel at the inner wall of the barrel is conveyed over the barrier flight and upstream into the adjacent melt channel. As described in more detail by Chung, in U.S. Pat. No. 4,000,884, and further developed by Medici, et al. in U.S. Pat. No. 6,056,430, the typical barrier melting section maximizes the amount of contact between the solid material and the heated inner surface of the barrel wall. As further explained by Medici, et al. in U.S. Pat. No. 6,056,430, conventional barrier melting sections provide for the solid material to be located on the xe2x80x9ctrail sidexe2x80x9d of the main flight, whereas the melt material is located at the xe2x80x9cpushxe2x80x9d side of the main flight
Therefore, it is important to move solids to the push side of the main flight in the metering section of the plasticating screw to provide higher pressure and shear which mixes and melts solids more effectively. Medici, Jr. et al. accomplishes the interchange as described in U.S. Pat. No. 6,056,430, by reversing the diameter and width of the primary and secondary flights at the terminal end of the mixing section. As an alternative, Medici, Jr., et al., uses a barrier flight having a short section of increased pitch at the terminal end of the barrier melt section which abruptly narrows the solids channel and thereby forces solid material over the barrier flight into the melt channel and on the pushing side of the primary flight before the metering section.
Although the configuration made by Medici, Jr., et al., in U.S. Pat. No. 6,056,430 may satisfy many general needs, thermal and composite mixing can be improved even more for various thermoplastic resin and polymer materials by including a more novel reorientation section between a multi-channel barrier melting section and the undulating metering section to better allow reorientation of solid and molten materials. Additional melting can take place via heat convection from the molten material. At the same time this invention permits greater temperature control to avoid the overheating or degradation of the resin. Further, the cost and time needed to manufacture the screw of the instant invention is reduced since intricate structure described by the Medici, Jr., et al., patent is eliminated in the instant invention and unique functional elements and parameters added and described.
Ultimately, the primary objective of the instant invention is to homogeneously mix select resins using an optimum combined barrier melting section and multi-channel undulating metering section, resulting in a completely molten material having uniform temperature, viscosity, color and composition at the terminal end of the metering section.
Medici, Jr. et al., confirms in U.S. Pat. No. 6,056,430 that multi-channel wave metering screws and barrier melting screws are well known. The present invention, like Medici, Jr., et al., modifies and combines the two technologies. Unlike Medici, Jr., et al., however, the instant invention eliminates the xe2x80x9cinterchangexe2x80x9d at the terminal end of the barrier melting section, and describes and claims, instead, a unique reorientation section between the barrier melting section and undulating metering section.
Throughout a plasticating screw, higher pressure and shear rates are obtained on the push side of the main flight since the main flight, as opposed to the barrier or secondary flights, provide reduced clearance with the inner barrel wall and an increased thread width, which in turn produces greater shear rates for the material being conveyed. Often, however, in conventional plasticating screws, the solids are located primarily on the trail side of the main flight instead of the push side of the main flight throughout. The present invention overcomes this disadvantage by providing a screw having a multi-channel barrier melting section and a reorientation section for transferring a major portion of the solids to the push side of the main flight before entering a undulating metering section.
The present invention is a plasticating apparatus comprising a barrel having an inlet and an outlet. A rotatable screw having a longitudinal axis is disposed within and cooperates with an inner wall of said barrel. The screw comprises a feed section, a barrier melting section, a reorientation section and a multi-channel undulating metering section located subsequently downstream along said screw axis. The feed section includes a main helical flight having a push side facing downstream and a trailing side facing upstream. A barrier flight is disposed in said barrier melting section, intermediate said main flight, and said barrier flight with the main flight divides the barrier melting section into a melt channel and solids channel extending helically side by side. The barrier flight includes helical threads with a diameter less than the diameter of the helical threads of said main flight, so that melt material flows over said barrier flight and into said melt channel. This allows melt material conveyed along the barrier melting section to be positioned adjacent the push side of said main flight.
To switch solids to the push side of the main flight in the present invention to take advantage of the higher shear thereof, the barrier flight terminates near the terminal end of the barrier melting section. Following the terminal end of the barrier melting section, the pitch of the main helical flight decreases and then traverses continuously through the reorientation section at least one turn or 360xc2x0 degrees along the longitudinal axis of the screw. Further, the solids channel and the melt channel in the barrier melting section merge into a substantially uniform reorientation channel at a location substantially coinciding with the decreased pitch of the main flight, thereby forcing solid plastic material conveyed into the reorientation section to move toward the push side of the main flight. After the reorientation section, the main flight traverses into the metering section with a secondary flight being introduced and disposed in said metering section intermediate the main flight whereby solid material conveyed along the metering section is positioned primarily adjacent the push side of the main flight.
Thus, one advantage of the present invention is that the melt material conveyed along the metering section is positioned primarily adjacent to the trail side of the main flight, allowing higher pressure and shear to be applied to the remaining solids. Since the secondary flight is undercut and thinner than the main flight, the secondary flight does not provide the shear and pressure of the main flight. By placing solids on the push side of the main flight in the metering section, the present invention applies higher pressure and shear rates to the solids.
Another advantage of the present invention is that the length of the reorientation section and depth of the reorientation channel may be easily changed to accommodate the various lengths and diameters of plasticating screws.